Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<p>After a function has been stored in a variable,
the variable can be used as a function:</p>

<p id="demo"></p>

<script>
var x = function (a, b) {return a * b};
document.getElementById("demo").innerHTML = x(4, 3);
</script>

</body>
</html>